Ultra low power source follower for capacitive sensor shield drivers

ABSTRACT

A source follower for a capacitive sensor device having a sense node and a shield node is provided. The source follower may include a transistor, and a switch array selectively coupling the transistor between the sense node and the shield node. The switch array may be configured to substantially disable current to the transistor during a first mode of operation, precharge the transistor during a second mode of operation, and enable the transistor to copy a sense node voltage to a shield node voltage during a third mode of operation.

TECHNICAL FIELD

The present disclosure relates generally to capacitive sensors, and moreparticularly, to ultra low power source followers for driving shieldedcapacitive touch sensors.

BACKGROUND

Capacitive sensor devices, or devices which measure capacitance or achange in capacitance, can be utilized in a wide variety of differentfields. This is because various data or parameters that are sought by agiven application can be derived based on capacitance or changestherein. For example, capacitive sensors can be used to detect touch,gesture or proximity input in human interface devices, to detectproximity of non-human physical objects, to detect presence and/orvolume of water or other liquids, to detect motion, doors and windowsfor security applications, and any other application that exhibits somechange in capacitance. While the following discussion will be directedto non-mechanical human interface devices for simplicity, it will beunderstood that the same discussions can be implemented for variousother uses and applications.

Among various non-mechanical human interface devices used today,capacitive sensor devices are often used to detect and measure touch orproximity input. Typically, a capacitive touch sensor implements analogcircuitry which measures changes in capacitance between two or moreelectrical wires caused by the proximity of a person's finger. Theresulting analog signal representing the change in capacitance isdigitized and post-processed to perform the preprogrammed task desiredby the user. Although modern capacitive sensors may be adequate, newchallenges surface when working with mobile or battery-operated devices.

Any battery-operated device shares the common goal of providing lowpower dissipation. While capacitive sensors may be modified to reducepower dissipation, the general trade-off or concern becomes anundesirable loss in the sensitivity of the capacitive sensor readoutsand/or the addition of undesirable noise. One efficient technique ofimplementing capacitive sensor readouts is to charge the capacitivesensor with a constant current for a fixed duration, and to read theresulting voltage across the charged capacitor at the end of the fixedduration to detect touch or proximity input. Moreover, two modes ofoperating in accordance with this technique are conventionally known, amutual-capacitance mode and a self-capacitance mode.

As shown in the prior art embodiment of FIG. 1 , the mutual-capacitancemode circuit 10 is composed of a grounded node 12 and a sense node 14,which collectively exhibit a combined capacitance that may be composedof a parasitic capacitance C_(SENSE,0) and a sense capacitanceC_(SENSE,F) reflective of touch or proximity input from a human finger16 for instance. If the mutual-capacitance mode circuit 10 isappropriately designed, such that the sense capacitance C_(SENSE,F) issufficiently greater than the parasitic capacitance C_(SENSE,0), touchinput can be efficiently and accurately detected by the capacitivesensor readout. However, when used to detect proximity input, themutual-capacitance mode circuit 10 drastically loses sensitivity andperforms rather poorly due to the sense capacitance C_(SENSE,F)diminishing to values decades smaller than the parasitic capacitanceC_(SENSE,0).

One solution to improve sensitivity is to implement the prior artself-capacitance mode circuit 18 shown in FIG. 2 , which is generallycomposed of a shield node 20 that is used to substantially surround thesense node 22 and driven with a copy of the voltage at the sense node22. This arrangement results in a parasitic capacitance CSENSE,0 that isonly due to stray capacitance and insignificantly small. Furthermore,although this arrangement creates a new cross-capacitance C2 between thesense node 22 and the shield node 20, this cross-capacitance C2 isnegated since the voltage applied to the shield node 20 copies ormatches the voltage at the sense node 22. The end result is increasedresolution of the sense capacitance C_(SENSE,F), and thereby improvedoverall sensitivity that is more suitable for detecting proximity inputat the sense node 22.

Although the self-capacitance mode circuit 18 of FIG. 2 is indeed animprovement, it can be further improved upon. Specifically, theself-capacitance mode circuit 18 copies the voltage between the sensenode 22 and the shield node 20 using a voltage buffer 24. A voltagebuffer 24 is often implemented by an operational amplifier connected inunity gain. However, such an operational amplifier is composed ofnumerous transistors, all of which can introduce unwanted noise. Whilethe noise generated by the operational amplifier can be reduced, suchnoise reduction then comes with increased power dissipation, which is asignificant drawback for battery-operated and/or mobile devices. Thus,it may be beneficial to modify the traditional self-capacitance modecircuit 18, and to substitute the operational amplifier with a bettermeans for driving shielded capacitive sensors operating inself-capacitance mode.

The present disclosure is directed at addressing one or more of thedeficiencies and disadvantages set forth above. However, it should beappreciated that the solution of any particular problem is not alimitation on the scope of this disclosure or of the attached claimsexcept to the extent expressly noted.

SUMMARY OF THE DISCLOSURE

In one aspect of the present disclosure, a source follower for acapacitive sensor device having a sense node and a shield node isprovided. The source follower may include a transistor, and a switcharray selectively coupling the transistor between the sense node and theshield node. The switch array may be configured to substantially disablecurrent to the transistor during a first mode of operation, prechargethe transistor during a second mode of operation, and enable thetransistor to copy a sense node voltage to a shield node voltage duringa third mode of operation.

In another aspect of the present disclosure, a capacitive sensor deviceis provided. The capacitive sensor device may include sense node havinga combined capacitance at least partially corresponding to proximityinput, a shield node partially surrounding the sense node, having ashield capacitance and forming a cross-capacitance with the sense node,and a source follower coupled between the sense node and the shieldnode. The source follower may be configured to substantially disablecurrent during a first mode of operation, precharge during a second modeof operation, and copy a sense voltage to a shield node voltage during athird mode of operation.

In yet another aspect of the present disclosure, a method of providing asource follower for a capacitive sensor device having a sense node and ashield node is provided. The method may include providing a transistorselectively coupled between the sense node and the shield node via aswitch array, enabling the switch array in a manner configured to definea first mode of operation, a second mode of operation, and a third modeof operation, disabling current to the transistor during the first modeof operation, precharging the transistor during the second mode ofoperation, and enabling the transistor to copy the sense node voltage toa shield node voltage during a third mode of operation.

These and other aspects and features will be more readily understoodwhen reading the following detailed description in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art embodiment of a capacitivesensor device arranged in a mutual-capacitance mode;

FIG. 2 is a schematic view of a prior art embodiment of a capacitivesensor device arranged in a self-capacitance mode;

FIG. 3 is a schematic view of one exemplary capacitive sensor devicearranged in a self-capacitance mode with a source follower of thepresent disclosure;

FIG. 4 is a schematic view of one exemplary embodiment of a sourcefollower of the present disclosure for driving a shield of a capacitivesensor device in self-capacitance mode;

FIG. 5 is a diagrammatic view of a timing diagram for operating thesource follower of FIG. 4 ;

FIG. 6 is a schematic view of the source follower of FIG. 4 in a firstmode of operation;

FIG. 7 is a schematic view of the source follower of FIG. 4 in a secondmode of operation;

FIG. 8 is a schematic view of the source follower of FIG. 4 in a thirdmode of operation; and

FIG. 9 is a flow diagram of one exemplary scheme or method of providingand controlling a source follower for a capacitive sensor device in aself-capacitance mode.

While the following detailed description is given with respect tocertain illustrative embodiments, it is to be understood that suchembodiments are not to be construed as limiting, but rather the presentdisclosure is entitled to a scope of protection consistent with allembodiments, modifications, alternative constructions, and equivalentsthereto.

DETAILED DESCRIPTION

Referring to FIG. 3 , one exemplary embodiment of a capacitive sensordevice 100 is diagrammatically provided. In general, the capacitivesensor device 100 may be used to monitor for changes in capacitanceresponsive to touch input, and more particularly to proximity input,such as via a human finger 102, and provide an analog output such as inthe form of a voltage on a sense node corresponding to the detectedtouch or proximity input. Moreover, the capacitive sensor device 100shown may be incorporated or implemented within mobile orbattery-operated devices, or any other form of electronic devicesconfigured to receive some form of touch or proximity input from a user,and perform a preprogrammed task in response to the touch or proximityinput. As shown, the capacitive sensor device 100 may generally includea sense node 104, a shield node 106, and a source follower 108 that isconstructed according to the present disclosure and designed to takeplace of a conventional voltage buffer.

More specifically, as shown in FIG. 3 , the sense node 104 may exhibit aparasitic capacitance C_(SENSE,0) with ground, and a sense capacitanceC_(SENSE,F) that is variably induced by proximity input from a finger102, or the like. The shield node 106 may be arranged to at leastpartially surround the sense node 104. For example, the sense node 104may include an upper surface 110 for interfacing with proximity inputand a lower surface 112 for interfacing with the shield node 106. Theshield node 106 may be positioned to be underneath, separated from andsubstantially surrounding the lower surface 112 of the sense node 104.In addition, the shield node 106 may exhibit a shield capacitance C₃with ground and form a cross-capacitance C₂ with the sense node 104.Furthermore, the source follower 108 may be coupled between the sensenode 104 and the shield node 106 and essentially copy the voltage at thesense node 104 to the shield node 106.

Turning now to FIG. 4 , the source follower 108 of FIG. 3 is disclosedin more detail. The source follower 108 may be designed to take place ofthe voltage buffer 24 implemented by an operational amplifier of theprior art self-capacitance mode circuit 18 in FIG. 2 and provide variousbenefits, such as reduced power dissipation and reduced noise, all whileincreasing sensitivity to levels sufficient for detecting proximityinput. As shown, the source follower 108 may further include atransistor 114 and a switch array 116 which, in the embodiment of FIG. 4may include switches S1-S7. The switch array 116 may generally beconfigured to selectively couple the transistor 114 between the sensenode 104 and the shield node 106 in a manner intended to essentiallycopy the voltage on the sense node 104 to the shield node 106.

Moreover, as illustrated for example in FIG. 5 , the switch array 116may configure the transistor 114 to operate in one of multiple modes ofoperation, such as to substantially disable current during a first modeof operation T_(OFF), precharge during a second mode of operationT_(PRECHARGE), and copy a sense voltage V_(SENSE) to a shield voltageV_(SHIELD) during a third mode of operation T_(CHARGE).

Still referring to FIG. 4 , the transistor 114 may be an N-typemetal-oxide semiconductor (NMOS), including a gate 118, a drain 120 anda source 122, and configured to interact with the plurality of switchesS1-S7 of the switch array 116. For instance, the switch array 116 may beconfigured to selectively couple the gate 118 between one or more of thesupply voltage 124, a source follower current I_(SF), and an inputcapacitance C_(IN). The switch array 116 may also be configured toselectively couple the drain 120 between the source follower currentI_(SF) and the supply voltage 124. Additionally, the switch array 116may be configured to selectively couple the source 122 between groundand the shield node 106. As shown in FIG. 5 , the switch array 116 maybe reiteratively and periodically enabled according to the first mode ofoperation T_(OFF), the second mode of operation T_(PRECHARGE), and thethird mode of operation T_(CHARGE), as collectively defined by aplurality of clock signals CK1-CK3.

Exemplary embodiments of the source follower 108 at different stages ofoperation are respectively illustrated in more detail in FIGS. 6-8 . Asshown in FIG. 6 , during the first mode of operation T_(OFF), all clockssignals CK1-CK3 are disabled or logically low, and the source follower108 does not consume any current. More particularly, switches S1, S5 andS7 are enabled or closed to short the gate 118 to the supply voltage124, and to short each of the sense node 104 and the shield node 106 toground. The drain 120 may be left floating and drawn to 0V since thetransistor 114 is enabled, and the source 122 may remain coupled toground. Furthermore, the gate 118 and the input capacitance C_(IN) areprecharged by the supply voltage 124. Correspondingly, as referenced inFIG. 5 , each of the sense voltage V_(SENSE) and the shield voltageV_(SHIELD) remains disabled or at a logical low value, such as 0V.

During the second mode of operation T_(PRECHARGE) in FIG. 7 , the firstand second clock signals CK1 and CK2 are enabled or logically high,while the third clock signal CK3 is disabled or logically low. Forinstance, the rising edge of the first clock signal CK1 disables oropens switch S1, while the rising edge of the second clock signal CK2enables or closes switches S3 and S4 to couple the gate 118 to thesource follower current I_(SF) and enable the transistor 114 to operateas a diode. Because the gate 118 was precharged with the supply voltage124, the voltage on the gate 118 may quickly settle to the value set bythe transistor 114 while the current at the drain 120 may be set by thesource follower current I_(SF). Furthermore, the input capacitanceC_(IN) is precharged to the voltage across the gate 118 and the source122, and the source 122 remains grounded. As shown in FIG. 5 , each ofthe sense voltage V_(SENSE) and the shield voltage V_(SHIELD) stillremains disabled or at a logical low value.

During the third mode of operation T_(CHARGE) shown in FIG. 8 , thefirst and third clock signals CK1 and CK3 are enabled or logically high,and the second clock signal CK2 is disabled or logically low, whichenables or closes switches S2 and S6. A fixed current I_(CHARGE) is thenapplied to the sense node 104 to charge the capacitive sensor device100, and the sense voltage V_(SENSE) increases as in FIG. 5 .Furthermore, the voltage at the gate 118 may shift from the sensevoltage V_(SENSE) with the voltage across the gate 118 and the source122 previously acquired during the second mode of operationT_(PRECHARGE), and the shield voltage V_(SHIELD) may shift down from thegate 118 by the voltage across the gate 118 and the source 122. When thesource follower current I_(SE) becomes approximately equal to thecurrent required to charge the shield capacitance C3, the shield voltageV_(SHIELD) substantially copies or becomes approximately equal to thesense voltage V_(SENSE) as shown in FIG. 5 .

According to the foregoing, as only one transistor 114 is employed indriving the shield node 106, the number of possible sources for noise aswell as the number of current paths drawing current from the supplyvoltage 124 are significantly reduced. Additionally, the source follower108 is completely off and no current is drawn when not in use.Substantially all of the current drawn, except for a minimal amount ofcurrent consumed during the second mode of operation T_(PRECHARGE), isallocated to charging the capacitive sensor device 100. Furthermore, inorder to obtain a unity gain from the sense node 104 to the shield node106 and to minimize the parasitic capacitance between the gate 118 andground, the shield of the input capacitance C_(IN) is coupled to theoutput of the source follower 108.

In alternative embodiments, switch S6 may be operated by the first clocksignal CK1 instead of the third clock signal CK3 to allow the fixedcurrent I_(CHARGE) to settle before the third mode of operationT_(CHARGE). In other alternatives, the source follower current I_(SF)may be implemented as a current digital-to-analog converter (DAC) suchthat the precharge current of the transistor 114 can be varied oradjusted according to the requirements of the shield node 106. Inrelated modifications, the current DAC may be varied or adjustedaccording to the output of a calibration algorithm configured tooptimize the performance of the capacitive sensor device 100, or anyreadout circuit associated therewith. While only certain embodiments,circuit arrangements and operating modes are depicted, it will beunderstood that other variations are possible without departing from thescope of the appended claims.

Turning now to FIG. 9 , one exemplary method 126 of providing andoperating a source follower 108 for a capacitive sensor device 100having a sense node 104 and a shield node 106 is provided. As shown, themethod 126 in block 126-1 may simply provide for a transistor 114 thatis selectively coupled between a sense node 104 and a shield node 106via a switch array 116 as shown for instance in FIG. 4 . The method 126in block 126-2 may define a reiterative and periodic cycle of events bywhich the switch array 116 may be operated. As shown in FIG. 5 , forexample, a plurality of clock signals CK1-CK3 may be generated whichcollectively enable and disable switches S1-S7 within the switch array116 in a manner configured to define a first mode of operation T_(OFF),a second mode of operation T_(PRECHARGE), and a third mode of operationT_(CHARGE).

Once the cycle is initiated, such as when the associated capacitivesensor device 100 is powered on, the method 126 may reiteratively cyclethrough the first mode of operation T_(OFF), the second mode ofoperation T_(PRECHARGE), and the third mode of operation T_(CHARGE), inthe sequence shown in FIG. 5 . In particular, during the Off Mode, orthe first mode of operation T_(OFF), the method 126 in block 126-3 maydisable current to the transistor 114. For instance, the clock signalsCK1-CK3 may operate the switch array 116 according to the embodimentsillustrated in FIGS. 5 and 6 discussed above. During the Precharge Mode,or the second mode of operation T_(PRECHARGE), the method 126 in block126-4 may precharge the transistor 114. More specifically, the clocksignals CK1-CK3 may operate the switch array 116 according to theembodiments illustrated in FIGS. 5 and 7 discussed above.

Still further, during the Charge Mode, or the third mode of operationT_(CHARGE), the method 126 in block 126-5 of FIG. 9 may enable thetransistor 114 so as to copy the voltage at the sense node 104 to theshield node 106. For example, the clock signals CK1-CK3 may beconfigured to operate the switch array 116 according to the embodimentsillustrated in FIGS. 5 and 8 discussed above. Once the Charge Mode, orthe third mode of operation T_(CHARGE) is complete, the method 126 mayloop back to the Off Mode, or the first mode of operation T_(OFF), andcontinue repeating the process until the capacitive sensor device 100 isdisabled or powered down. It will be understood that the method 126shown in FIG. 9 is demonstrative of only one exemplary set of processesconfigured to provide and enable the capacitive sensor device 100discussed further above, and that other variations of the method 126will be apparent to those of ordinary skill in the art.

From the foregoing, it will be appreciated that while only certainembodiments have been set forth for the purposes of illustration,alternatives and modifications will be apparent from the abovedescription to those skilled in the art. These and other alternativesare considered equivalents and within the spirit and scope of thisdisclosure and the appended claims.

What is claimed is:
 1. A method comprising: providing a source followercoupled between a sense node of a capacitive sensor device and a shieldnode of the capacitive sensor device via a switch array of the sourcefollower, wherein the source follower includes a transistor and a switcharray, wherein the switch array selectively couples the transistorbetween the sense node and the shield node, wherein the switch arrayselectively couples a gate of the transistor between one or more of asupply voltage of the capacitive sensor device, a source followercurrent of the capacitive sensor device, and an input capacitance of thecapacitive sensor device, wherein the switch array selectively couples adrain of the transistor between the supply voltage and the sourcefollower current, and wherein the switch array selectively couples asource of the transistor between ground of the capacitive sensor deviceand the shield node to copy a sense node voltage of the sense node tothe shield node; enabling the switch array in a manner configured todefine a first mode of operation, a second mode of operation, and athird mode of operation; disabling current to the transistor during thefirst mode of operation, wherein, during the first mode of operation,the gate is shorted to the supply voltage and the sense node and theshield node is shorted to the ground; precharging the transistor duringthe second mode of operation, wherein, during the second mode ofoperation, the gate is coupled to the source follower current; andenabling the transistor to cause the sense node voltage of the sensenode to be substantially equal to a shield node voltage of the shieldnode during a third mode of operation.
 2. The method of claim 1, whereinthe switch array is enabled according to a plurality of clock signalsenabled to collectively define the first mode of operation, the secondmode of operation, and the third mode of operation.
 3. The method ofclaim 2, wherein the clock signals are periodically reiterated tosequentially cycle through the first mode of operation, the second modeof operation, and the third mode of operation.
 4. The method of claim 2,wherein, during the first mode of operation, the gate and the inputcapacitance are precharged to the supply voltage, the drain is floating,and the source is coupled to the ground.
 5. The method of claim 2,wherein, during the second mode of operation, the gate is coupled to thesource follower current and allowed to settle, the drain is coupled tothe source follower current, the source is coupled to the ground, andthe input capacitance is precharged to a gate-source voltage of thetransistor.
 6. The method of claim 2, wherein, during the third mode ofoperation, the sense node is coupled to a fixed supply current, thedrain is coupled to the supply voltage, and the source is coupled to theshield node.
 7. The method of claim 1, further comprising: disabling thecurrent to the transistor during the first mode of operation afterenabling the transistor to cause the sense node voltage to besubstantially equal to the shield node voltage during the third mode ofoperation.
 8. A capacitive sensor device comprising: a sense node; ashield node; and a source follower coupled between the sense node andthe shield node, wherein the source follower includes a transistor and aswitch array, wherein the switch array selectively couples thetransistor between the sense node and the shield node, wherein theswitch array selectively couples a gate of the transistor between one ormore of a supply voltage of the capacitive sensor device, a sourcefollower current of the capacitive sensor device, and an inputcapacitance of the capacitive sensor device, wherein the switch arrayselectively couples a drain of the transistor between the supply voltageand the source follower current, wherein the switch array selectivelycouples a source of the transistor between ground of the capacitivesensor device and the shield node to copy a sense node voltage of thesense node to the shield node, wherein, during a first mode ofoperation, the gate is shorted to the supply voltage and the sense nodeand the shield node is shorted to the ground, wherein, during a secondmode of operation, the gate is coupled to the source follower current,and wherein, during a third mode of operation, the sense node voltage ofthe sense node is substantially equal to a shield node voltage of theshield node.
 9. The capacitive sensor device of claim 8, wherein theswitch array is enabled according to a plurality of clock signalsenabled to collectively define the first mode of operation, the secondmode of operation, and the third mode of operation.
 10. The capacitivesensor device of claim 9, wherein the clock signals are periodicallyreiterated to sequentially cycle through the first mode of operation,the second mode of operation, and the third mode of operation.
 11. Thecapacitive sensor device of claim 9, wherein, during the first mode ofoperation, the gate and the input capacitance are precharged to thesupply voltage, the drain is floating, and the source is coupled to theground.
 12. The capacitive sensor device of claim 9, wherein, during thesecond mode of operation, the gate is coupled to the source followercurrent and allowed to settle, the drain is coupled to the sourcefollower current, the source is coupled to the ground, and the inputcapacitance is precharged to a gate-source voltage of the transistor.13. The capacitive sensor device of claim 9, wherein, during the thirdmode of operation, the sense node is coupled to a fixed supply current,the drain is coupled to the supply voltage, and the source is coupled tothe shield node.
 14. The capacitive sensor device of claim 8, whereinthe current to the transistor is disabled during the first mode ofoperation after enabling the transistor to cause the sense node voltageto be substantially equal to the shield node voltage during the thirdmode of operation.
 15. A source follower comprising: a transistor; and aswitch array, wherein the source follower is coupled between a sensenode of a capacitive sensor device and a shield node of the capacitivesensor device, wherein the switch array selectively couples thetransistor between the sense node and the shield node, wherein theswitch array selectively couples a gate of the transistor between one ormore of a supply voltage of the capacitive sensor device, a sourcefollower current of the capacitive sensor device, and an inputcapacitance of the capacitive sensor device, wherein the switch arrayselectively couples a drain of the transistor between the supply voltageand the source follower current, wherein the switch array selectivelycouples a source of the transistor between ground of the capacitivesensor device and the shield node to copy a sense node voltage of thesense node to the shield node, wherein, during a first mode ofoperation, the gate is shorted to the supply voltage and the sense nodeand the shield node is shorted to the ground, wherein, during a secondmode of operation, the gate is coupled to the source follower current,and wherein, during a third mode of operation, the sense node voltage ofthe sense node is substantially equal to a shield node voltage of theshield node.
 16. The source follower of claim 15, wherein the switcharray is enabled according to a plurality of clock signals enabled tocollectively define the first mode of operation, the second mode ofoperation, and the third mode of operation.
 17. The source follower ofclaim 16, wherein the clock signals are periodically reiterated tosequentially cycle through the first mode of operation, the second modeof operation, and the third mode of operation.
 18. The source followerof claim 16, wherein, during the first mode of operation, the gate andthe input capacitance are precharged to the supply voltage, the drain isfloating, and the source is coupled to the ground.
 19. The sourcefollower of claim 16, wherein, during the second mode of operation, thegate is coupled to the source follower current and allowed to settle,the drain is coupled to the source follower current, the source iscoupled to the ground, and the input capacitance is precharged to agate-source voltage of the transistor.
 20. The source follower of claim16, wherein, during the third mode of operation, the sense node iscoupled to a fixed supply current, the drain is coupled to the supplyvoltage, and the source is coupled to the shield node.